Parasympathetic Nervous System: A complete guide

Heard of fight or flight mode? What about rest and digest? What is our parasympathetic nervous system, how does it work, and how is it activated? How does it affect our brain? Our organs? Our hormones and stress levels? What disorders come from our parasympathetic nervous system? Find out this and more in this article!

What is the Parasympathetic Nervous System?

Our nervous system is made up of three nerve types: motor nerves, sensory nerves, and autonomic nerves. Our motor nerves help our brain command our muscles to move, or not move, around. Our sensory nerves help us see, smell, taste, hear and feel. The autonomic nerves assist in regulating our organs and their function. The autonomic nervous system (ANS) is divided into two parts: the sympathetic nervous system (SNS) and the parasympathetic nervous system (PNS). The parasympathetic nervous system is the part of our involuntary nervous system whose purpose is to slow down the heart rate, increase glandular and intestinal activity, and relax the sphincter muscles. 

The term parasympathetic derives from old English. Para– meaning “alongside” and sympathetic because the nerves run alongside the sympathetic nerves from the sympathetic nervous system.

Structure of the Parasympathetic Nervous System

The parasympathetic nervous system is controlled by nerves from the cranium, vagus nerve, and the spinal cord. The cranial nerves originate in the brain and come from the central nervous system (CNS) and join together at the parasympathetic ganglia: the ciliary, pterygopalatine, otic, or the submandibular. From these ganglia, the parasympathetic nerves go their way to get tissues via the trigeminal branches which contain the ophthalmic nerve, maxillary nerve, and the mandibular nerve.

The vagus nerve, coming from the Greek word for wandering, goes from the top of the brain all the way down to the bottom of the spinal cord. It begins in the brainstem and goes down to the tongue, vocal chords, lungs, heart, and our vital organs. It sends out chemical messages, known as neurotransmitters. The main neurotransmitter sent with regard to the parasympathetic nervous system is called acetylcholine. The intention of this neurotransmitter is meant to cause changes throughout the body.

The spinal cord plays a major role in the parasympathetic nervous system because the nerve cell bodies reside in the lateral gray horn matter where the neurons join together close to the supply organ.

To put it simply, the parasympathetic nervous system is a slower system that moves alongside longer pathways. Preganglionic fibers from our spinal cord or medulla propel ganglia close to the target organ. The fibers and the organ will create a synapse, a joining together, which will create the desired response of homeostasis.

Functions of the Parasympathetic Nervous System

The Parasympathetic Nervous System is responsible for the stimulation of activities that occur in the body when it is resting during the “feed and breed” or “rest and digest” state. This means that our body temperature cools, our heart rate slows, and our bodies begin to be in a more resting, less-active state. 

The most important function of the PNS is that it allows our bodies to function properly. However, it does more than just that. The PNS also helps regulate sexual activity, sweating, salivation, urination, the flow of tears, bowel movements, and our overall ability to digest well.

The parasympathetic nervous system is involved in our sensations and how we feel emotionally, too. It was theorized that sensory stimulation (for example, acupuncture) affects the cardiovascular system by way of the autonomic nervous system- especially the parasympathetic part of the ANS. It’s been found that using acupuncture, as well as other sensation stimuli, in a healthy person is associated with improved activity in both the sympathetic and the parasympathetic nervous systems. Emotionally, there is a theory that speculates that our emotions and our autonomic nervous system are well connected to each other. This theory, The Polyvagal Theory, makes sense. When our parasympathetic nervous system is activated, we feel emotions and feelings like tiredness, being relaxed, or hungry.

Sexually, the ANS, and subsequently the PNS, play a large role in the activity. It begins by the ANS regulating the cardiovascular system- both physiologically and pathophysiologically. There is evidence that there is a difference between genders and their effects from both male and female hormones in the autonomic nervous system. Within the human species, there is a larger amount of sympathetic mediated sexual responses in males and a larger amount of parasympathetic mediated sexual responses in females.

The sympathetic and parasympathetic nervous systems don’t often work together at the same time. However, one of the times when they do is during sexual activity. In men, having an erection doesn’t mean that one’s heart rate goes up just because they have an erection- that’s thanks to the parasympathetic nervous system. Within the human sexual response, there are four parts: the erection (parasympathetic nervous system), the emission (sympathetic nervous system), the ejaculation (somatic), and the resolution (sympathetic nervous system). Typically after sexual activity, one fees tired. After the resolution happens, there is a refractory period. This refractory period is the time when one feels tired and is controlled by the parasympathetic nervous system. However, it is also thought that one feels tired after sex due to brain chemistry and the neurotransmitters that are released. Such as serotonin and prolactin.

The parasympathetic nervous system and sympathetic nervous system go together hand-in-hand. Both systems originate in the spinal cord and from there they fork off into the body. While people don’t have any real self-control over their parasympathetic nervous system, they do have more control over their sympathetic nervous system. While one system works, the other can’t work. This is due in part to the fact that they contradict each other. One is flight or fight while the other is rest and digest, or breed and feed.

The parasympathetic nerves are nerves in our autonomic nervous system that allow us to have daily functions like digestion and sleeping. It’s known for its ability to “rest and digest”. The parasympathetic nervous system helps us maintain a balance within our body. Our muscles can relax and our heartbeat decreases.

The sympathetic nerves are nerves in our autonomic nervous system that are linked to our body’s flight or fight response. These nerves help our bodies to prepare for stress, like a threat or an injury, by contracting our muscles and speeding up our heartbeat.

When we feel in danger, our bodies redirect the blood flow from our parasympathetic nerve functions, like digestion, to the sympathetic nerve functions, like muscle contractions. That’s why when we feel severe anxiety, it’s harder to fall asleep, stay asleep, and have normal bowel movements. They compliment each other.

How does a parasympathetic nervous system affect the brain?

Our hippocampus, the area of the brain that is used for spatial learning and memory, tries to turn off our physiologic stress responses- fight or flight mode. The hippocampus needs our prefrontal cortex to be able to turn off this response. It does so by activating the parasympathetic nervous system in order to balance out the physiologic stress response, which comes from the sympathetic nervous system. The hippocampus and prefrontal cortex also help balance the sympathetic and parasympathetic nervous systems within our autonomic nervous system.

When the parasympathetic nervous system is activated, our brain has short-term and long-term benefits.

The short-term benefits can be a release of de-stressing hormones like prolactin, oxytocin, vasopressin, and acetylcholine. Our brains get a chance to relax and feel more at peace. This gives our brains time to process better. Our prefrontal lobe is the area of the brain that is in charge of our short-term memory. A team of researchers from the University of California found that our autonomic nervous system promotes the process of converting information and memories from short-term to long-term during the sleeping process. The study found that a nap can improve one’s memory by 33%!

Long-term benefits include an improved memory, higher levels of growth hormones, better sleep, and aids in relieving tension headaches. It’s also thought that frequent stimulation of the vagus nerve can help tame allergic responses. Our long-term memory comes from information passing through the hippocampus several times.

How does a parasympathetic nervous system affect our organs?

• The heart beats slower and relaxes which in turn makes the heart rate and blood pressure lower
• The pupils in our eyes get smaller
• Our digestive system gets a leg up and works better because extra blood is sent to the stomach and intestines.
• Our breathing slows down and our bronchial tubes in our lungs narrow
• Our blood vessels get wider, a process known as vasodilation, which helps lower blood pressure.

How does a parasympathetic nervous system affect hormones and stress?

The main hormone used in the parasympathetic nervous system is the neurotransmitter acetylcholine. When we sleep, acetylcholine helps lower our heart rates. In order for our parasympathetic system to activate, our levels of adrenaline and cortisol, our stress hormones, need to be low. When the vagus nerve is stimulated, and thus the parasympathetic nervous system is activated, our brain releases an array of anti-stress hormones: prolactin, oxytocin, and vasopressin.

The hormones released while the parasympathetic nervous system is active are meant to calm you down and destress you. Stress management techniques are aimed to induce a parasympathetic state. Because the parasympathetic nerves relax our muscles and are designed to let us “rest and digest”, we are automatically destressing when the system is activated.

Health Issues and Disorders of the Parasympathetic Nervous System

There are many issues and disorders that can arise in our nervous system. Some of the most common ones include:

• Infections such as polio and meningitis
• Functional disorders such as epilepsy, dizziness, and headaches
• Degeneration of Parkinson’s disease, Alzheimer’s, or multiple sclerosis

• Structural disorders such as any major injury to the spinal cord, spinal cord tumors, or peripheral neuropathy
• Disruption in blood flow
• Autoimmune disorders
• Vascular disorders such as a stroke or hemorrhage

How can we activate our parasympathetic nervous system?

Science says that by stimulating our parasympathetic nervous system our stress levels will go down. There are several exercises that can help to stimulate the vagus nerve and activate the parasympathetic nervous system. It’s important to stimulate the vagus nerve because when it’s stimulated, it counters the sympathetic nervous system (our fight-or-flight response)

• A quick exercise when having a panic attack, during an exam or big presentation, or trying to get sleepy, breathe in for four seconds, hold your breath for seven seconds, and exhale for eight seconds. Repeat twice. This causes our body’s autonomic nervous system to shift from the sympathetic nervous system or the parasympathetic one.
• Try deep breathing, also known as belly breathing. Studies have shown that 10 minutes of deep breathing exercises a day can calm one’s overall reactions. The entire idea of deep breathing is to expand the belly/abdomen rather than the chest. While sitting up straight, take a deep breath into your belly while counting to ten. Then exhale while counting to ten. Try to reduce the number of breaths from 10-14 a minute to 5-7 per minute.
• Some studies have shown that having a cold water facial after exercising can help stimulate the vagus nerve.
• Meditation will also help activate our parasympathetic system because it concentrates on breathing and relaxation.

Let us know how you activate your parasympathetic nervous system in the comments below!

Anna Bohren

Anna is a freelance writer who is passionate about translation, psychology, and how the world works.